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Are femtocell interference issues fully appreciated? John Spindler, vice president of product management for ADC’s wireless business, investigates…
Recently, femtocells seem to have caught the imagination of a large number of people, including the media and other industry insiders. As with many technologies that receive a lot of attention, there appears to be a gap between the hype and what femtocells can actually deliver. Some people are talking about femtocells as if they are the saviour of in-building cellular communications. Unfortunately, it is a bit more complicated than that. In this article, we will look at femtocells’ real capabilities in relation to the problems currently being experienced by those trying to implement effective and reliable in-building wireless communications networks. The femtocell was originally designed to deliver coverage and capacity over a relatively small area, similar to a Wi-Fi Access Point (AP). If wireless LAN architects want to cover a larger area, they can deploy multiple APs to facilitate this. The idea is that carriers or enterprises can deploy multiple femtocells in the same way to cover large buildings. But it is not so simple as that. The mobile network is not a private, unlicensed wireless LAN, and using multiple femtocells to provide coverage within an enterprise can cause a lot of problems. One of the main issues that affect those trying to scale coverage with multiple femtocells is figuring out how to reduce interference with cell towers and other femtocells. When such interference occurs, network performance is inevitably reduced. Avoiding interference with the outside macro network is the more challenging problem to address. To ensure sufficient coverage along the inside building perimeter, femtocells are placed close to the outer walls. However, due to these cells’ proximity to the building’s exterior, a certain amount of interference between those cells and the macro network is unavoidable. This interference is never appreciated by wireless carriers and it is the enterprise’s responsibility to rectify the problem. To minimise femtocell interference, some carriers are considering deploying femtocells on a different RF frequency from macro cellular networks, but spectrum acquisition can be costly. Rather than using multiple femtocells, a different way to approach the coverage question is to use the signal from a single femtocell (or larger ‘super femto’) and distribute it throughout the required area using a Distributed Antenna System (DAS). Using this arrangement, interference in coverage areas is no longer a problem as all RF channels are distributed to each antenna in the DAS, effectively creating one large cell. Interference with external networks can be reduced by designing the DAS network to only direct signal inwards toward the building core. In a multi-femtocell environment, additional issues can include hand-offs and over–provisioning. In such environments the user’s handset has to hand off the connection from one cell to the next depending on the user’s location in the building. This drains handset battery life (and thus reduces the usefulness of the device) and it occurs with far greater frequency than in the macro network due to the femtocells’ small cell size. In buildings covered by a DAS there are no handoffs as the entire area is essentially one large cell. Another challenge is signal dominance. To function properly, an in-building system must establish signal dominance in order to reduce hunting between the indoor and outdoor signal sources. Such hunting also reduces battery life and creates a poor calling experience. But it is difficult to establish a dominant signal source with femtocells due to their very low output power. With a DAS, however, it is easy to deliver enough power through the antennae to create a dominant signal source and minimise hunting. Over-provisioning is yet another challenge. This refers to the need for high user density locations (conference rooms, staff restaurants and other communal areas) to be over-provisioned with femtocells in order to provide enough capacity for peak usage times. During low usage this investment in extra femtocells is effectively wasted. Over-provisioning issues are also more effectively managed with a DAS because all of the system’s capacity is available to every antenna within the coverage area. If required, larger enterprises can use picocells or a micro cell base station to increase capacity rather than deploying multiple femtocells. Finally, it is not entirely clear exactly who takes responsibility for quality of service and management of an in-building femtocell network. Do enterprise IT managers really want to take on this challenge when centralised femtocells located in a data centre are far easier to manage, and the DAS itself requires nothing more than monitoring for antenna failures? While femtocells seem to be all the rage at present, mobile network architects need to be aware that in certain environments they may cause as many problems as they are likely to solve. There are many logistical factors which businesses must consider if they want a solution that will meet their requirements. |
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